We report on the covalent functionalization of carbon nanotubes with tetramanganese complexes. The process is based on ligand exchange with carboxylic groups on the preliminarily oxidized tubes and does not fundamentally affect the antiferromagnetic coupling between the manganese ions of the complex. We present detailed analysis of the oxidation process and demonstrate that the degree of functionalization directly relates to the density of carboxylic groups. The coverage of a carbon nanotube can be decreased enough to enable fabrication of electronic devices incorporating an individual molecule.